1. Field of the Invention
Certain features, aspects and advantages of the present invention relate to a method for controlling a pressure balance between two chambers of a shock absorber, in which a damping medium pressure in a return chamber preferably does not fall below a preselected minimum pressure, whereby the likelihood of cavitation is greatly reduced. As a result of the method, energy is stored in the shock absorber so that a pressure is built up in a resilient device, preferably in the form of an accumulator, that is disposed in the return chamber. Thus, a resilient function is created in the resilient device, which initially during the compression stroke increases the pressure in the return chamber.
Certain features, aspects and advantages of the present invention also relate to a pressurized shock absorber in which a resilient device is positioned within or directly adjacent to a return chamber, the resilient device comprising a pressurizing member or pressurizing medium that acts in an inner volume delimited from the return chamber.
2. Description of the Related Art
A shock absorber operates by adjusting the pressure ratio between the pressure exerted upon the damping medium in the compression chamber and the pressure exerted upon the damping medium in the return chamber respectively. A high pressure gives a higher rigidity. In other words, a greater force is required to compress the damping medium when operating under a higher pressure. The piston that separates the compression and the return chamber can be provided with flow-adjusting valves or can be leak-tight when used in combination with externally mounted valves that adjust the flow between the chambers.
The pressure drop over the piston determines the pressure ratio and the pressure drop can be altered dynamically by having a system pressure act upon the damping medium. The system pressure can be determined by a pressurizing member mounted in or on the shock absorber body. The pressurizing member is connected to and pressurizes either just the compression chamber or both the compression and the return chamber. The pressurizing member is designed to receive the pressure medium that is displaced by the piston rod, to absorb the changes in damping medium volume caused by temperature differences, and to generate a certain basic pressure (i.e., the system pressure) in the shock absorber. The damping medium flow between the pressurizing member and both or one of the damping chambers can be adjustable with one or more adjustable valves, hereinafter referred to as cylinder valves.
In a compression stroke in an ideal shock absorber with a cylinder valve, the pressure in the return chamber is constantly equal to the system pressure. The counterpressure that is created with the aid of the cylinder valve therefore compensates for the reduction in pressure in the return chamber which is brought about by the pressure drop over the piston.
In a real shock absorber, an ideal compression stroke is impossible because the rigidity in the return chamber is higher than the rigidity in the compression chamber when the shock absorber approaches the rebounded state. The pressure in the compression chamber does not build up as fast as the pressure in the return chamber falls, with the result that it is not possible to use the cylinder valve to increase the pressure in the return chamber. With too low a pressure in the return chamber, the risk of cavitation increases and cavitation causes a loss of damping forces.
Examples of previously known attempts to solve this problem can be found in US2004134730 or in the Applicant's own patent EP0601982. EP0322608 further shows an embodiment in which the damping medium is conducted both through the piston and through a duct outside the damping chamber depending on the stroke rate. In the case of certain rapid compression movements, however, it is difficult for the damping medium to pass through this duct, which means that the pressure in the return chamber nonetheless falls below that in the compression chamber.
In document DE10052789, a shock absorber is shown that solves another problem, namely the adjustment of the damping flow between the return chamber and a space designed to absorb the piston rod displacement and any differences in damping medium volume due to temperature changes, for example. The flow of damping medium from the return chamber into the space is adjusted with an adjustable damping valve. The valve plate of the adjustable damping valve is pretensioned with a resilient pressurizing device disposed in a sleeve-shaped part around the outer strut of the shock absorber. By pressurizing the valve plate in varying amounts, the damping medium flows through the valve only once certain damping movement speeds are attained.